In chemical synthesis and related processes, conventional heating devices typically use conduction (e.g., hot plates) or convection (e.g., ovens) to heat reaction vessels, reagents, solvents, and the like. Under some circumstances, these kinds of devices can be slow and inefficient. Moreover, maintaining the reactants at a temperature set point can be difficult using conduction or convection methods, and quick temperature changes arc almost impossible.
Conversely, the use of microwaves, which heat many materials (including many reagents) directly, can speed some processes (including chemical reactions) several orders of magnitude. This not only reduces reaction time, but also results in less product degradation--a result of the interactive nature of microwave heating. In some cases, reactions facilitated by microwave devices proceed at a lower temperature, leading to cleaner chemistry and less arduous work-up of the final product. In addition, microwave energy is selective--it couples readily with polar molecules--thereby transferring heat instantaneously. This allows for controllable field conditions producing high-energy density that can then be modulated according to the needs of the reaction.
Many conventional microwave devices, however, have certain limitations. For example, microwave devices are typically designed to include a rigid cavity. This facilitates the containment of stray radiation, but limits the usable reaction vessels to sizes and shapes that can fit inside a given cavity, and requires that the vessels be formed of microwave transparent materials. Moreover, heating efficiency within such cavities tends to be higher for larger loads and less efficient for smaller loads. Heating smaller quantities within such devices is less than ideal. Measuring temperatures within these cavities is complicated. Another problem associated with microwave cavities is the need for cavity doors (and often windows) so that reactions vessels can be placed in the cavities and thc reaction progress reaction may be monitored. This introduces safety concerns, and thus necessitates specially designed seals to prevent stray microwave radiation from exiting the cavity.
Alternatively, typical microwave cavities are rarely designed ordinary laboratory glassware. Thus, either such cavities or the glassware must be modified before it can be used in typical devices. Both types of modifications can be inconvenient, time-consuming, and expensive.
Furthermore, the typical microwave cavity makes adding or removing components or reagents quite difficult. Stated differently, conventional microwave cavity devices tend to be more convenient for reactions in which the components can simply be added to a vessel and heated. For more complex reactions in which components must be added and removed as the reaction (or reactions) proceed, cavity systems must be combined with rather complex arrangements of tubes and valves. In other cases, a cavity simply cannot accommodate the equipment required to carry out certain reactions.
Some microwave devices use a waveguide fitted with an antenna (or "probe") to deliver radiation in the absence of a conventional cavity. Such devices essentially transmit microwave energy to the outside of a container to facilitate the reaction of reactants contained therein, e.g., Matusiewicz, Development of a High Pressure/Temperature Focused Microwave Heated Teflon Bomb for Sample Preparation, Anal. Chem. 1994, 66, 751-755. Nevertheless, the microwave energy delivered in this manner typically fails to penetrate far into the solution. In addition, probes that emit radiation outside of an enclosed cavity generally require some form of radiation shielding. Thus, such probe embodiments have limited practical use and tend to be employed mainly in the medical field. In this context, however, the applied power is typically relatively lower, i.e., medical devices tend to use low power (occasionally 100 watts, but usually much less and typically only a few) at a frequency of 915 megahertz, which has a preferred penetration depth in human tissue. Moreover, because microwave medical probes are typically employed inside a body, stray radiation is absorbed by the body tissues, making additional shielding unnecessary.